Offshore platforms are generally fabricated in a harbor or on a shore location and are then towed to a marine site where they are tipped on end and lowered so that the legs of the platforms rest on the ocean floor. Traditionally, the legs are hollow structures so that pilings can be driven downwardly through the legs and into subterranean formations below the ocean floor. Generally, the ends of the platform legs are open to permit piling to be driven therethrough. However, when the platforms are placed in position, silt and other debris enter the lower ends of the legs before the pilings are driven therethrough. Since the annulus between the piling and legs is usually filled with cement or grout, it is most desirable to exclude as much foreign material as possible from the legs. It is therefore desired to provide a closure structure for the platform legs and one which will easily sever when the piling is driven through the platform legs.